Hydraulic pumps are commonly used for many purposes and in many different applications. Vehicles, such as, for example, highway trucks and off-highway work machines, commonly include hydraulic pumps that are driven by an engine in the vehicle to generate a flow of pressurized fluid. The pressurized fluid may be used for any of a number of purposes during the operation of the vehicle. A highway truck, for example, may use pressurized fluid to operate a fuel injection system or a braking system. A work machine, for example, may use pressurized fluid to propel the machine around a work site or to move a work implement.
A hydraulic pump typically includes a pumping element that applies work to an operating fluid to increase the pressure of the fluid. In one type of hydraulic pump, the pumping element includes a series of piston that are disposed in cylinders. The pistons are driven through a reciprocal movement within the cylinders to compress the operating fluid. The pumping element may be fixed displacement, where the stroke length of the pistons is constant. Alternatively, the pumping element may be variable displacement, where the stroke length of the pistons may be varied.
As shown in U.S. Pat. No. 6,035,828 to Anderson et al., a fixed displacement pump may include a metering device that allows the output flow rate of the pump to be varied. In the described system, the metering device includes a series of metering sleeves that are disposed around a series of pistons. The metering sleeves are configured to selectively block a passageway that provides a fluid connection with a compression chamber in the cylinder. When the passageway is open, operating fluid may flow from the compression chamber through the passageway to thereby prevent pressurization of the operating fluid during the compression stroke of the piston. The rate at which the pump generates pressurized fluid may be controlled by varying the position of the metering sleeves. The rate of pressurized fluid generation may be increased by covering the passageway for a greater portion of the compression stroke. The rate of pressurized fluid generation may be decreased by leaving the passageway open for a greater portion of the compression stroke.
The metering sleeves have a close tolerance relative to the outer surface of the pistons to minimize the amount of fluid that leaks from the passageway. It is expected that some operating fluid will leak from the passageway through the clearance between the metering sleeve and the piston surface. This fluid may be used to lubricate the surfaces of the metering sleeve and piston, which may facilitate movement between the metering sleeve and piston. Under some operating conditions, such as when the engine is cold, the viscosity of the operating fluid may be relatively high. The high viscosity of the fluid results in a greater drag between the metering sleeve and the piston. This increases the force required to move the metering sleeve relative to the piston. Accordingly, accurately controlling the position of the metering sleeve relative to the piston may be more difficult when the engine is cold.
In addition, when the metering sleeves are covering the spill ports, an inner surface of the metering sleeves will be exposed to the pressurized fluid within the compression chamber. Particularly in high pressure systems, the pressurized fluid exerts a significant force on the inner surface of the metering sleeve. Over time, this force may cause the metering sleeve to swell or deform. The swelling or deformation of the metering sleeve may increase the clearance between the metering sleeve and the piston. The increased clearance may lead to an increase in the amount of fluid that leaks from the passageway, which may decrease the volumetric efficiency of the pump.
The pumping element of the present disclosure solves one or more of the problems set forth above.